This invention relates to a communication system and more particularly to a spread spectrum or pseudo-noise communication system.
A spread spectrum or pseudo-noise communication system derives its name from a coding technique that translates the usual narrow band information spectrum into a wide band (spread) spectrum which resembles that of noise. The common ingredient of this class of communication system is the coder. A typical means of instrumenting the coder is with an n-stage shift register driven by a master clock. The stages of the shift register are interconnected by logical feedback elements. This produces an output consisting of a seemingly random binary sequence which is actually periodic. The number of bits in the sequence N=2.sup.n -1 (in a typical embodiment) and the code period is N times the master clock period. By changing the logical connections between stages, different binary sequences are formed of which there are of the order of 2.sup.n. These unique combinations of binary "1" and binary "0" become, in effect, discrete addresses for a communication system of this type.
The coded transmission is recognized by the intended receiver through a correlation process. This involves the generation of an identical coded waveform within the receiver which is compared with the received signal. When the two identical coded waveforms are exactly aligned, the original narrow band information signal is reconstituted and can be detected by conventional techniques. Should the two waveforms be displaced an interval of time equal to the clock period, the correlator output becomes approximately zero. The necessity for close code synchronization is characteristic of this spread spectrum systems of the prior art and the present invention. This involves the use of acquisition circuitry which sweeps the phase between a transmitter-receiver code pair to bring them into coarse phase alignment followed by a track mode to maintain fine phase alignment.
A transmission with a coded waveform possessing a different binary sequence than the one generated in the receiver represents an interference between user sets. The rejection of an interfering signal is equal to the protection ratio which is the ratio of the spread spectrum bandwidth to the information bandwidth. For instance, a four megacycle (mc) spread spectrum system operating with a four kilocycle (kc) voice band has a protection ratio of 30 db (decibel). This means that the effective power of an interfering signal is reduced on the average by the protection ratio.
There are two spread spectrum systems known in the prior art, namely, phase reversal (or higher or pseudo-random phase modulation) and frequency hopping. These two types of systems have all the characteristics described for pseudo-noise systems but differ in one important respect which is their operation in an interfering environment.
The phase reversal system applies the coding technique to vary the phase of the voice modulated carrier in a pseudo-random (noise) fashion. Carrier phase is selected as either 0.degree. or 180.degree. depending on the binary state of the coded sequence. The system uses a common carrier frequency for all subscriber sets such that the interfering signals are present for the entire duration of any one desired transmission. The sum total of interfering power is reduced by the protection ratio.
In a phase reversal system, the transmitter carrier oscillator is modulated by voice signals. After amplification and perhaps frequency multiplication, the signal appears at the input to a balanced modulator. For a voice bandwidth of 4 kc, the total RF (radio frequency) spectrum spread will be 8 kc. The pseudo-random or pseudo-noise coding is accomplished in the balanced modulator by selecting the carrier phase as either 0.degree. or 180.degree. depending on whether a binary "1" or a binary "0" is present in the coding waveform. The resultant spread spectrum signal is then amplified and transmitted.
The received coded waveform is multiplied by the locally generated coded waveform in a balanced modulator. The resultant signal is passed through conventional IF (intermediate frequency) stages and detected by standard techniques with subsequent filtering to the voice bandwidth. This technique of multiplication and filtering is one way of instrumenting a correlation detector.
Since the coded waveforms in the transmitter and receiver have to be aligned within a reasonable time, sweep circuitry is employed in the receiver. Sweeping is accomplished by speeding up or slowing down the receiver clock with respect to the transmitter clock frequency. An acquisition peak detector will determine when coarse correlation occurs and stops the clock sweep. Once coarse phase alignment is obtained, fine synchronization is maintained by switch over to a tracking loop.
The frequency hopping type communication system applies the binary sequence code to select the frequency of transmission in a pseudo-random fashion. Each of the selected frequencies is voice modulated prior to transmission. The spread spectrum is achieved by hopping among x different frequencies. For instance, a one megacycle spread is obtained by hopping among 100 different frequencies spaced 10 kilocycles apart. The nature of the spectrum is also determined by the rate at which these frequencies are chosen. A fast hopping system implies a hopping rate in the order of the information bandwidth while a slow hopping rate is a rate much lower than the information bandwidth.
Interference from other subscriber sets is not continuous but occurs only during the intervals of time when both are on the same frequency. On the average, this will occur 1/x of the time between two subscriber sets. In this limited sense, the protection ratio of the frequency hop system can be considered equal to the number of hops x.
The frequency hop system has a block diagram that is almost identical to the phase reversal system described above. The frequency hop system is instrumented with a similar code generator driving a selection matrix such that one out of a group of 2.sup.x frequencies is chosen for transmission in a pseudo-random fashion. The process of acquiring a desired signal involved sweeping the receiver bit clock until the presence of a signal signifies coarse alignment with the received signal. The sweep circuit is disabled and the bit clock maintains close synchronization with the received signal through a tracking loop. This process is basically identical to the phase reversal system.